1. Field of the Invention
The present invention generally relates to voltage regulation for microprocessors and more particularly to a voltage regulation steering circuit for a microprocessor.
2. Description of the Related Art
The importance of voltage regulation for microprocessors has grown as processor voltages continue to drop and processor power continues to increase. In personal computers, voltage regulation is especially needed during voltage transition periods for a microprocessor. With certain personal computer systems, processor core voltage changes have occurred during a power-up sequence for a microprocessor, a transition of a computer system into a low power mode, and a transition of a computer system from a low power mode.
A primary component of voltage regulation circuitry in a computer system has been a voltage controller, also termed a switching regulator controller. A voltage controller typically receives a programmable voltage identification (VID) code from a microprocessor to be decoded by an internal digital-to-analog converter (DAC) of the voltage controller. Based on the particular VID code, the voltage controller provides pulse width modulated (PWM) voltage outputs to a set of logic drivers for driving a set of power transistors to supply a certain core voltage to the microprocessor.
To protect the microprocessor, power supply and overall power system of a personal computer system, a voltage controller has employed over-voltage, under-voltage and over-current protection. Such protection typically involves the voltage controller monitoring a feedback signal indicating the processor core voltage and comparing the processor core voltage to the VID code. If the processor core voltage is greater than an over-voltage threshold defined by a predetermined percentage above the VID code, then an over-voltage condition is detected. Detection of an over-voltage condition may result in a shutdown of the voltage controller or a substantial reduction of the output voltage. After a substantial reduction of the output voltage, the microprocessor may not recover, such that the computer system becomes trapped in a low power mode. If the processor core voltage is less than an under-voltage threshold defined by a predetermined percentage below the VID code, then an under-voltage condition is detected. Detection of an under-voltage condition may trigger a power-good (PWRGD) signal from the power supply to be deasserted to indicate the processor core voltage is outside of a proper operating range, causing the system to reset.
There are critical reliability issues with voltage regulation circuitry during voltage transitions for a microprocessor. For a power-up initialization sequence of a microprocessor, a soft-start function is typically used by voltage regulation circuitry to move a processor core voltage up to an established intermediate voltage from which there is essentially an instantaneous voltage transition from the intermediate voltage to a desired processor core voltage. In the case of the AMD Athlon™ processor available from Advanced Micro Devices, an acceptable intermediate or low power voltage is 1.300 volts and the desired processor core voltage is 1.750 volts. In the case of the AMD Duron™ processor also available from Advanced Micro Devices, an acceptable intermediate or low power voltage is 1.300 volts and the desired processor core voltage is 1.600 volts. A sudden change in a processor core voltage introduces large inrush currents that produce undue stress on power supply output capacitors, input capacitors of voltage regulation circuitry, output capacitors of voltage regulation circuitry, and power transistors of voltage regulation circuitry. Such undue stress may significantly reduce the life of voltage regulation circuitry. Abrupt changes in a processor core voltage may also result in “false” over-voltage, under-voltage and over-current conditions detected by the voltage controller which may shut off the voltage regulation circuitry. Since inrush currents also affect the power supply, another risk of instantaneous processor core voltage changes is that the power supply may be shut down due to excessive output current slew rates or additional voltage perturbations.